I want "instant" not "fast"!
Fat will be lost in any diet where the calorific intake is lower than expenditure. However these "crash" diets and low-energy diets will result in a muscle loss in addition to the fat burned. It is estimated that the maximum fat a typical human body can burn in any one week is 2 pounds. This means that any weight loss over and above 2 lbs in a week will be mainly muscle or water.
Crash dieting slows down your metabolism . If you don't eat enough food your ancient survival instincts take over and the body enters "starvation mode". In this mode the metabolic rate slows to conserve energy and thus the speed at which weight is lost is reduced. Thus ultimately less fat is burned using "crash" methods.
In addition the body has lost muscle during this crash phase. Muscle burns calories and the more muscle, the more calories and burned. Crash dieting therefore delivers the double whammy of a drop in metabolic weight and a drop in calorie burning muscle tissue.
This is the reason why so many dieters who rely upon fad diets regain weight quickly after losing it. The only safe reliable and long term way to reduce weight is to do it over a period of time.
A gradual reduction in calories maintains muscle tissue, burns only fat and allows you to maintain a healthy lower weight in the future.